(231a) Influence of Supra-Molecular Structure and Storage Conditions on the Caking of Powders

Caking is frequently observed while handling and processing of powders. The caking intensity can be quantified by applying various shear or compression tests or by visual assessment in combination with a pre-defined scale. One of the most reproducible tests is the ring shear test which is especially suitable for quantifying the inter-particle adhesion in the early stage of the time consolidation process. Once a stable powder cake is obtained also uni-axial compression tests are suitable for measuring the caking intensity. Considering that for the daily industrial practice such methods are often too complicated and that rather seldom a geometrical defined powder cake is obtained, it is useful to combine them with a simple visual semi-quantitative assessment of the powder. All these methods have been used in the current study for investigating the influence of supra-molecular structure and storage conditions on the caking process.       

Solids can have different supra-molecular structures like various crystalline forms or amorphous molecular arrangements. Depending on their supra-molecular structure and storage conditions like relative humidity of the surrounding air and the temperature different mechanisms can cause caking of powders: Increasing Van der Waals forces due to particle deformation, dissolution and re-crystallization of water soluble substances, melting and solidification of crystalline materials and sintering of amorphous solids. However, like shown in the current study, knowing the adhesion mechanism makes it possible to predict the caking kinetics.

In a first series of experiments the caking of water-soluble crystalline substances like sodium chloride has been investigated. Sodium chloride dissolves if the relative humidity of the surrounding air exceeds 73-75%. This can be the case if moisture migrates into the powder bulk or water condensates within the powder pores due to decreasing temperature. In consequence the sodium chloride crystals dissolve partly at their surface while forming a saturated salt solution. This solution generates liquid bridges between neighboring particles which are then transformed into crystals bridges upon drying (see fig. 1).

This phenomenon is well known. However, in the current study it will be shown that also capillary condensation can cause a caking of densified fine sodium chloride below the critical relative humidity of 73%. Capillary condensation seems to be a suitable explanation for the observed changes in relative humidity during compaction and the measured increase in tensile strength of compacted sodium chloride particles.  

Furthermore, the caking of water insoluble crystalline substances like fat powders was investigated. Caking of such powders can be observed if melted at least partially by forming interparticle bridges (see fig. 2).

Since industrial fat powders are mostly a mix of different molecules and polymorph crystal forms they have no defined melting point. Applying differential scanning calorimetry the amount of molten fat was measured depending on the temperature. It was investigated which amount of material has to be liquid to lead to a significant caking of a fat powder containing different molecules and crystal forms. 

In contrast to crystalline materials, amorphous solids do not melt at a defined temperature nor do they dissolve at a defined relative humidity. Such substances can be plasticized by low molecular substances like water or glycerol which migrate into their molecular structure. Increasing plasticizer content and/or increasing temperature decrease the viscosity of the solid and thus lead to sintering of powder particles (see fig. 3).

Combining the sinter equation from Rumpf et al. [1976] with the superposition principle the intensity of caking can be predicted (see equation 1) by calculating the theoretical sinter bridge diameter x [Palzer, 2005].

(1)

x is the sinter bridge diameter, a the particle diameter, g the surface tension, F_t the force with which the particles are pressed together, T the temperature, T_g the glass transition temperature, t the time, t_max the time available for the sinter process and C and B are constants.

According to the superposition principle the relaxation time and thus also the viscosity of an amorphous solid depends on its temperature and its glass transition temperature. The glass transition temperature which can be considered as a kind of softening temperature, itself is a function of the plasticizer (e.g. water) content of the solid. In the current study the caking of different amorphous food powders like tomato powder, milk powder and dextrose syrup has been investigated depending on the storage temperature and humidity. The glass transition temperature of these powders was measured by differential scanning calorimetry for varying water content of the solid. The powders were stored under different temperatures and humidity for a defined time. Following, caking was quantified by using ring shear tests and visual assessment of the stored samples. In parallel, the caking process was predicted by applying the sinter kinetics and the superposition principle and by calculating the theoretical sinter bridge diameter. Some of the results are shown in fig. 4 and 5.

Figure 4: Unconfined yield strength measured by ring shear tests versus calculated ratio between sinter bridge x and particle diameter a

Fig. 5: Caking grade (pre-defined caking scale) and calculated ratio between sinter bridge x and particle diameter a for dextrose syrup DE21 depending on the storage time and the climate conditions

It was found, that exceeding a ratio between sinter bridge diameter and particle diameter of 0.15 a significant caking (unconfined yield strength exceeding by definition 5000 Pa) was obtained. Thus, calculating the theoretical sinter bridge diameter seems to be a valid method to estimate the intensity of caking of amorphous water miscible solids.     

Obviously it can be concluded, that while knowing the supra-molecular structure and physical parameters of the solid material like melting point, glass transition temperature and its solubility in water the storage conditions at which caking is likely to occur can be estimated.

Literature

Palzer, S. (2005): Desired and undesired agglomeration of amorphous powders. Proceedings of the 8th international symposium on agglomeration. March 2005. Bangkok/Thailand. p. 251-264

Rumpf, H.; Sommer, K.; Steier, K. (1976): Mechanismen der Haftkraftverstärkung bei der Partikel-haftung durch plastisches Verformen, Sintern und viskoelastisches Fliessen. Chem.-Ing.-Tech., 48 No.4, p. 300-307